The present invention is generally concerned with telecommunication systems.
The present invention is more particularly concerned with telecommunication systems using ATM (Asynchronous Transfer Mode). ATM is a commonly used standard for transmission of high bit rate data in telecommunication systems, which is based on an asynchronous time multiplexing of packets of fixed length called cells.
Telecommunication systems using ATM may be modelized by an ATM layer and an interface layer, or ATM Adaptation Layer (AAL), between the ATM layer and users.
A specific ATM Adaptation Layer has been provided for low bit rate applications such as in particular mobile communication systems, especially third generation mobile communication systems (such as in particular UMTS: xe2x80x9cUniversal Mobile telecommunication Systemxe2x80x9d).
Such a specific layer, or AAL2, is defined in ITU-T Recommendation 1.363.2. AAL2 is subdivided into a Service Specific Convergence Sublayer (SSCS) and a Common Part Sublayer (CPS) which allows several low bit rate connections to share a same ATM connection, by multiplexing several short packets called CPS packets into a same ATM cell.
AAL2 layer thus receives different incoming streams of low bit rate data units which, besides, may have different rates and/or lengths, in particular depending on the associated type of traffic (voice, data, images, signalling . . . etc.). Each incoming data unit stream is converted into an incoming short packet stream, by packetizing data units into short packets (or CPS packets). These different incoming short packet streams are multiplexed into a single outgoing short packet stream to be packed into a same ATM connection.
To be efficient, the process carried out at CPS level should simultaneously achieve a number of objectives, in particular:
this process should be bandwidth efficient, i.e. ATM cells should be sent with as least padding as possible,
this process should meet quality of service requirements, in particular it should introduce as least delay variations as possible,
this process should meet different quality of service requirements for the different types of traffic to be multiplexed; in particular voice traffic is time sensitive and requires stringent delay variations objectives, while data or signalling traffic are usually tolerant to transfer delay variations.
A special cause of delay variations is the processing of long data units (i.e. in practice of data units of length higher than 45 octets ).
Long data units are currently segmented prior to packetization into short packets. In this context, WO 97/48251 considers the case of two types of traffics of different priorities (i.e. voice and data traffic) and teaches to interrupt segmentation and packetisation of a long data unit (i.e. data traffic) upon the arrival of a short and more prioritary user data unit (i.e. voice traffic), in order to reduce the cell delay variations that might result of the processing of such long data units on such high priority traffic.
However this reference does not provide any solution in the general case of any number of incoming streams, and of any types of traffic carried by the different incoming streams; in particular it does not provide any solution to the problem of delay variations that might result, in the case of a plurality of incoming streams carrying long data units, of the processing of long data units of a given stream, on each of the other incoming streams.
The present invention in particular enables to avoid such drawbacks.
An object of the present invention is therefore a method for generating ATM cells for low bit rate applications, said method including:
a segmentation and packetization step, for converting a plurality of incoming data unit streams into a plurality of incoming short packet streams, said step including a segmentation of long data units prior to the packetisation of data units into short packets,
a multiplexing and scheduling step, for multiplexing said plurality of incoming short packet streams into a single outgoing short packet stream to be packed into a same ATM connection, and for scheduling the different incoming short packet streams into said single outgoing short pocket stream, in a way that short packets corresponding to long data units of different incoming data unit streams con be interleaved in said single outgoing short pocket stream.
According to another object of this invention, said step of multiplexing and scheduling is further carried out in a way that short packets corresponding to long data units of different lengths and/or rates can be interleaved in said outgoing short packet stream as a function of their respective lengths and/or rates.
According to another object of this invention, scheduling of a given incoming short pocket stream is determined so that short pockets of this stream are spaced of Ti in said single outgoing short packet stream, where Ti is the ratio between the transmission time interval, or data unit repetition period of the corresponding incoming data unit stream, and the number of short packets after segmentation of a data unit of this incoming data unit stream.
According to another object of this invention, said step of multiplexing and scheduling is further carried out in a way that short packets corresponding to short data units can be inserted in said outgoing short packet stream, as a function of their respective length and/or rate.
According to another object of this invention, said method further includes a step of scheduling ATM cell transmission times in a way as to keep ATM cell spacing as constant as possible, and a step of packing said single outgoing short packet stream into a some ATM connection having the thus scheduled ATM cell transmission times.
According to another object of this invention, said ATM cell spacing is kept as close as possible to a cell rote negociated for the corresponding ATM connection.
Depending on the type of traffic agreement for the ATM connection, and according to various embodiments, said cell rate may be:
a PCR (Peak Cell Rate) in the case of service category of DBR (Deterministic Bit Rate) or CBR (Constant Bit Rate) type,
a BCR (Block Cell Rate) in the case of service category of ABT (ATM Block Transfer) type,
a ACR (Allowed Cell Rate) in the case of service category of ABR (Available Bit Rate) type.
According to another embodiment, said cell rate may be re-negociated, to optimise resource utilisation in the network.
According to another embodiment, no ATM cell is sent when there is no data available from said single outgoing short packet stream, and said method includes a further step of referencing said scheduling step with respect to the next availability of data from said single outgoing short packet stream.
Another object of the present invention is a device for generating ATM cells for low bit rate applications, said device including
segmentation and packetization means, for converting a plurality of incoming data unit streams into a plurality of incoming short packet streams, said means including means for segmenting long data units prior to packetizing data units into short packets,
multiplexing and scheduling means, for multiplexing said plurality of incoming short packet streams into a single outgoing short packet stream to be packed into a same ATM connection, and for scheduling the different incoming short packet streams into said single outgoing short packet stream, in a way that short packets corresponding to long data units of different incoming data unit streams can be interleaved in said single outgoing short packet stream.
The present invention also has for its object an entity such as a base station (or Node B in UMTS) for a mobile radiocommunication network, comprising such a device.
The present invention also has for its object an entity such as a base station controller (or Radio Network Controller, or RNC, in UMTS) for a mobile radiocommunication network, comprising such a device.